Is there a shuttle service to downtown? Do you have to pay in advance or can you pay and jump on when you arrive? Thank you.

There is the Downtown Indy Express, which I have used several times in years past and am planning to use this year. You can set up for a specific time, or buy a ticket for the next available one at the counter when you arrive.

Downtown Indy Express Shuttle Service - GO Express Travel

The lines can be long for the bus, the bus can run very late.  And expect that the line may well be ignored at first in favor of anyone with a scheduled ticket, then fill as best they can with the walk-ins.

It will be very hot, it will be very crowded, luggage space is at a premium.  Myself, I usually would use the bus, but given that level of crowded of previous years, I think I'll leave it to others this year, so hopefully if others feel the same, the lines won't be as bad.  Of course, now it is suspended, probably a wise move.  We might get van-shares here soon.

You can also reserve a scheduled ticket on your phone while walking through the airport when it is available.

In prior years, the pick up point is through the rental car area across the street from the baggage claim, then far to the right side.  Not the left, or partially to the right.  All the way to the right, be sure to check what line you are in.

You can also take the IndyGo 8 Bus.
Several years ago my hotel was by the airport, so I took the hotel complimentary airport shuttle to the airport and caught the IndyGo 8 bus. Way cheaper than the shuttle bus, but a little longer. I told the driver I wanted to go to the convention center and she told me she would let me know when we got there. This was before the new Transit Center at Delaware St and Washington St, though. Thats about a six block walk, but its not too bad.

The Indy route 8 bus (coming from the airport) still has a stop right in front of the convention center.   I've taken it a number of times in the past.  If you're not in a big hurry, it's an inexpensive option to get downtown.

Cool. I couldn't really tell from the bus schedule. Its not a bad ride.